In recent years there have been major changes in the use of fasteners. One of these changes has been a shift to the use of high strength bolts and studs. Although these cost more per fastener than lower strength bolts or studs of the same size the overall cost of the completed joint is reduced. This lower cost results because fewer or smaller high strength fasteners are required and because design and manufacturing changes in the joint components become possible when fewer or smaller fasteners are used, leading to further cost savings.
Another significant change in fastener use has been the increasing acceptance by industry of the practice of tightening bolts into the plastic range, tightening them until they yield. This acceptance has been prompted by two advantages which accrue from tightening to yield. First, the tensions produced in the bolts are much more uniform than the tensions obtained when bolts are tightened within the elastic range because of the much greater influence that variation in the friction conditions has on the tension of bolts tightened within the elastic range. Second, tightening bolts into the plastic range utilizes the maximum possible strength of the bolt and thus allows the use of smaller bolts or fewer bolts of the same size.
There are three conditions which must be well controlled if tightening bolts to yield is to produce satisfactory results. First, the yield points of the bolts as manufactured must be kept within a specified range of tension variation. Second, the wrenching system must be capable of identifying accurately the yield point of each bolt so that the tightening can be halted when yield is reached. Third, care must be taken that no bolt is tightened into the plastic range so far that either the bolt breaks during tightening or the beginning of fracture is initiated at some point within the bolt with the result that the bolt breaks later when the equipment is in service.
Several wrenching systems have been developed for tightening bolts to yield. Angle control wrenching systems, also called turn-of-the-nut systems, operate by tightening the bolt through a specified angle which is large enough to bring the bolt into the plastic region but not so large there is danger that the bolt will break or that the beginning of fracture will be initiated.
Other wrenching systems make use of the fact that the torque required to tighten a bolt is proportional to the tension existing in the bolt and, therefore, the shape of the torque-rotation curve is proportional to the shape of the tension-rotation curve. Such wrenching systems incorporate transducers which measure torque and angle of tightening rotation, and a small on-board computer continuously monitors the outputs of these transducers and computes the torque-rotation gradient (slope). Tightening to yield is accomplished by programming the computer to stop tightening when the torque-rotation slope has dropped to some fraction (say, two-thirds) of the slope computed during the elastic tightening phase.
Angle control wrenching systems work relatively well with low strength bolts because these bolts have a relatively large ductility; that is, after beginning to yield plastically during tightening these bolts can be rotated through a relatively large angle before fracture occurs. This large ductility allows the low strength bolt to be tightened through a specified angle certainly large enough to bring the bolt tension beyond the beginning of plastic yield but not so large as to bring it to the point of maximum tension.
However, these angle control wrenching systems work less well with high strength bolts. These bolts have limited ductility and this means the specified angle through which the bolt is rotated cannot be large. Also, high strength bolts do not have a sharp yield point and this leads to uncertainty as to the minimum limit to be put on the specified angle of rotation, a situation complicated by the fact that the compression stiffnesses of the joint components must be taken into account.
Continuously monitoring torque-rotation gradient wrenching systems work relatively well with low strength bolts. The large ductility of these bolts allows the tightening to be stopped when the torque-rotation slope has dropped to a relatively small fraction (say, one-third) of the elastic tightening phase slope without danger that the point of maximum tension has been reached. Being able to program the wrench to stop tightening at a smaller fraction of the elastic tightening phase of the torque-rotation slope enhances the accuracy of the preload tensions obtained because of two factors. First, a point of lower slope in the torque-rotation curve also is a point of lower slope in the tension-rotation curve and thus variations in the rotation angle at which tightening is stopped leads to smaller variations in the preload tension. Second, a point at which the torque-rotation slope is a large fraction of the elastic tightening phase slope is harder to identify correctly because the stick-slip nature of friction behavior causes random variations in individual torque readings and these lead to variations in torque-rotation increments which might be interpreted mistakenly as changes in the slope itself, the effect of this being greater the smaller the difference between the elastic slope and the target slope. This difficulty is reduced by "smoothing" of the torque-rotation data, but slope measurement errors remain to cause variation in the preload tensions obtained.
The preload tensions produced in high strength bolts by continuously monitoring torque-rotation gradient wrenching systems are subject to greater variation because of the random data variations discussed above. Because these bolts have limited ductility and do not have a sharp yield point the target slope for stopping tightening must be a relatively large fraction of the elastic tightening phase slope. Additional fluctuations in the preload tensions result from variations in the hardness of manufactured bolts. The allowable hardness variation specified for high strength bolts means that the tensile strength, and thus the yield strength, can vary over a considerable range, by as much as 15 to 20%.
I previously have invented and patented a process for reducing the range of variation in preload tensions produced in high strength bolts tightened to yield by a continuously monitoring torque-rotation gradient wrenching system. This process, which is set forth in my U.S. Pat. Nos. 4,035,858 and 4,078,273, consists of adding to the manufacturing process a step which consists of work hardening each bolt by subjecting it to a tensile force of given magnitude. The bolts so treated all will have the same yield point level irrespective of their individual hardnesses and, further, when being tightened each bolt will have a discontinuity in the slope of its torque-rotation curve at the same level of preload tension. There will be a corresponding discontinuity in the slope of the torque-rotation curve which a continuously monitoring torque-rotation gradient wrenching system will be able to detect with accuracy.
One deficiency with my pre-use work hardening process of U.S. Pat. Nos. 4,035,858 and 4,078,273 is that it adds a relatively complicated step to the manufacturing process and thus increases the cost of the bolts. A second deficiency is that this process does not increase the ductility of high strength bolts. An increase in the bolt ductility would allow the use of a larger maximum angle limit for the specified angle employed by angle control wrenching systems and, also, permit a greater number of reuses of a bolt tightened to yield.
Because of these deficiencies I began to search for a more economical way to provide high strength fasteners with a definite yield point and, at the same time, increase their ductility substantially. As will be evident from the following description of my invention I have been successful in this search.